2390
Organometallics 1993,12, 239Q-2392
Optical Rotation Measurements of Organometallic Compounds: Caveats and Recommended Procedures Michael A. Dewey and J. A. Gladysz' Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received December 31, 1992 Summary: There are many possible sources of error in determining specific rotations, [aITx, of chiral, nonracemic organometallic compounds (e.g., purity, sample mass, solution volume, temperature). These are systematically analyzed, and recommended procedures are outlined. Protocolsfor calculating95 % confidence limits are given. The first optically active transition metal complexes were reported in 1911.l However, during the last decade there has been dramatically increasing interest in chiral, nonracemic organometallic complexes.2 This derives from a variety of exciting new applications in mechanistic studies, Ziegler-Natta catalysis, and enantioselective syntheses. Consequently, many new optically active organometallic complexes are being synthesized and characterized by their specific rotations, [crITh. Although polarimetry is one of the oldest instrumental techniques in chemistry? is is not a familiar one for most inorganic and organometallic chemists, Further, none of the modern methods for determining enantiomeric excesses (eel-such as chiral NMR shift reagents and chromatographF7-have rendered (tothe regret of many)8polarimetryobsolete. Hence, in this note we compile some caveats and recommendations based upon our extensive experience with optically active chiral organorhenium comple~es.~ We believe these will aid researchers who are new to this field. 1. Purity of Compounds. For many chiral molecules, enantiomerically pure samplesdo not crystallize as readily as racemic samples.1° Thus, the former can be much more difficult to purify. A procedure that gives a racemic compound in analytically pure form is by no means guaranteed to give the corresponding enantiomerically pure compound in analytically pure form. Hence, microanalyses (and other solid state properties such as melting points) should be redetermined. Any impurity, or subsequently generated thermal decomposition or oxidation product, can greatly affect the observed rotation a. (1) (a) Werner, A. Ber. Deutsch. Chem. Ges. 1911, 44, 1887. (b) Kauffman,G. B.; Lindley, E. V., Jr. J. Chem. Educ. 1974,51,424.
(2) (a) See articles published in: Organometallic Compounds and Optical Actioity, Brunner, H., Ed.; Journalof OrganometallicChemistry; Elsevier Sequoia: Lausanne, Switzerland, 1989; Vol. 370. (b) Noyori, R. CHEMTECH 1992,22,360. (c) Caldarelli, J. L.; White, P. S.;Templeton, J. L. J. Am. Chem. Soc. 1992,114,10097. (d) Nugent, W. A.; RajanBabu, T. V.;Burk, M. J. Science 1993,259, 479. (3) Biot, J. B. Mem. Acad. Sci. Toulouee 1817,2, 41. (4)Fra~er,R. R. In Asymmetric Synthesis, Morrison, J. D., Ed.; Academic Press: New York, 1983; Vol. 1, Chapter 9. (5) Ramsden, J. A.; Garner, C. M.; Gladysz, J. A. Organometallics 1991,10,1631.
(6)Schurig, V. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Prese: New York, 1983; Vol. 1, Chapter 5. (7) Pirkle, W. H.; Finn, J. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1983; Vol. 1, Chapter 6. (8) Meyers, A. I. Colorado State University, personal communication. (9) boseou, F.; OConnor, E. J.; Garner, C. M.; Quirb,Mhdez, N.; Fern& ez, J. M.; Patton, A. T.; Ramden, J. A.; Gladysz, J. A. Inorg. Synth. 1992,29, 211. (IO)Brock, C. P.; Schweizer, W. B.; Dunitz, J. D. J. Am. Chem. SOC.
2
1991,113,9811.
To illustrate the potential effect of a small amount of sample oxidation, we added 1mol % of the radical cation [ ( T ~ - C S H S ) ~ F ~ I 'to + Pa FCHzCl2 ~solution of the methyl complex (+)-(S)-(s6-CsHs)Re(N0)(PPh3)(CH3), with [alSaq2s= 180 f 4 O (199% ee).9 This compound normally exhibits excellent configurational stability. However, over the course of 1 h under an inert atmosphere, [a],=j,# dropped to 29 f 5 O ! Brunner has reported similar phenomena with optically active, diastereomeric cyclopentadienyliron complexes.ll Hence, if it proves necessary to determine [ a l Tfor ~ a sample that is not analytically pure (a situation we have occasionally found unavoidable),12it should be explicitly stated. Extra handling precautions should be taken with thermally or oxidatively sensitive materials. 2. Minimization of Weighing Errors. By some criteria, concentrated samples are desirable in Spectroscopy. Errors in weighing and the measurement of weak absorbances can be minimized. However, many organometallic compounds strongly absorb visible light. If insufficient light is transmitted through the polarimeter cell, especially at the 589-nm wavelength most commonly utilized, readings become impossible. Thus, the specific rotations of many chiral organometallic compoundsare of necessity determined on dilute solutions-often such that the amount of substrate in the cell is less than 1 mg. A related consideration is that most enantiomerically pure organometallic compounds give [a1T~values of >looo. Hence, it is important to follow a protocol that ensures a minimum of three significant digits. All of these concerns can be addressed as follows. First, weight out at least a 0.0100-g sample. On standard analytical balances, this quantity can be assigned a 95% confidencelimit, A(weight) or A(w),130f f0.0002g.~~ Next, prepare a standard solution in a volumetric flask, for which 95% confidence limits, A(vo1ume)or A(v),13 have been determined.lS for lo-, 25-, and 50-mL volumetric flasks, values are f0.02,10.03, and f0.05 mL, respecti~ely.~~ These quantities will be propagated through to 95 % confidence limits for [ a l T values ~ as described below. 3. Temperature Control. The volume of a solution is temperature dependent and, consequently, so is the concentration of any solute. An example worked in a textl3 shows that the change in density of water between 18and (11)Brunner, H.; Fisch, K.; Jones, P. G.; Salbeck, J. Angew. Chem., Znt. Ed. Engl. 1989,28,1521. (12) Agbossou, F:; Ramsden, J. A.; Huang, Y.-H.; Arif, A. M.; Gladysz, J. A. Organometalkcs 1992,11, 693. (13) Abbreviations follow the standard in: Shoemaker, D. P.; Garland, C. W. Experiments in Physical Chemietry, 5th ed.; McGraw-Hilk New York, 1989; p 55. The symbol A(x) designates a 95% confidence interval and signifies that x A(%) has a 95% probability of containing the true value of the quantity x. This book also utilizes polarimetry as the basis for an amusing anecdote regarding error analysis. The treatment presented in this paper was developed independently. (14) This value is calculated from the repeated weighing of a standard sample on our analytical balance and corresponds to two standard deviations ( 2 ~ ) .
Q276-7333/93/2312-239Q$O4.QOJQ 0 1993 American Chemical Society
Notes
Organometallics, Vol. 12, No. 6, 1993 2391
25 "C should only affect a by a factor of 0.998625/0.997075, (iii) or
